1. Field of the Invention
The present invention relates to a method of correcting processing errors and a method of forming overlay mark. More particularly, the present invention relates to a method of correcting lithographic process and a method of forming overlay mark capable of preventing errors in alignment measurement when the overlay mark is used for alignment in semiconductor fabrication.
2. Description of the Related Art
In general, aside from a proper control of the critical dimension, the yield of a wafer after a photolithograhic operation also depends on alignment accuracy. Therefore, the measurement of alignment accuracy or overlay error measurement in semiconductor fabrication is very important. An overlay mark is an auxiliary tool for measuring overlay errors. The overlay mark is a means of determining if a patterned photoresist layer produced after a photolithographic process is accurately aligned with a previous film pattern. Particularly, after an aluminum conductive line layer has been globally deposited over a wafer and the aluminum layer etched using a photoresist layer in the fabrication of metallic interconnect, the positions of alignment marks and overlay marks are often measured and compared. This ensures the aluminum lines are accurately aligned with the contact or plug. Furthermore, if there is any shifting in the alignment, the next photoresist layer for defining another aluminum line layer can be modified to compensate for the error.
Typically, the overlay mark is designed to dispose on the corner or peripheral region of chips on the wafer and are formed during the process of forming the metallic interconnects. Hence, if there is some error in the process of fabricating the metallic interconnects, relative positions of the overlay marks on the wafer will also be affected. In other words, the measured values of various overlay marks will differ.
As an example, aluminum lines are formed in a sputtering process. During the sputtering process, plasma production is closely related to the production of gaseous plasma ions (for example, argon ions). In other words, the probability of bombardment between high-energy electron and gaseous plasma atoms directly affects the sputtering process. To increase the probability of ionization of the gaseous plasma atoms (the so-called sputtering yield), the mean free path of the electrons within the plasma is preferably increased. At present, the most commonly used method for increasing the mean free path is to deploy an additional magnetron device. Typically, this means that a rotatable magnetron device is installed over a target within a plasma reaction chamber. Thus, through the magnetic field created by the magnetron device, charge particles within the reaction chamber are activated to follow a spiraling motion, thereby increasing the probability of collision. Because electrons are light particles, they have a very small radius of gyration and are mostly constrained to move within a short distance from the magnetic lines. In other words, electrons will mostly spiral near the magnetic field and increase the plasma ion density as well as ion bombardment frequency there.
Because the magnet above the target rotates around a center, the bombarded surface of the target will develop concentric openings after a period of time. Since the target surface is no longer a flat surface, a portion of the sputtered metallic atoms may collide with the sidewalls of the openings and lead to a change in the sputtering rate between the central and peripheral portion of the target. When the sputtered atoms reach the recess hole or openings on the wafer, the sidewalls facing the central direction and the peripheral direction of the target will have different sputtered film thickness. Moreover, as the sputtering operation is continued, the amount of variation in the concentric opening will increase and the magnetic field strength at the sputtering location will intensify. Ultimately, asymmetric deposition of the sputtered film is amplified as shown in FIGS. 1A and 1B.
In the meantime, gaseous plasma ions subjected to the magnetic field from the magnetron device will bombard the target surface at a small biased angle so that quantity of sputtered metallic atoms in the reflecting direction is larger than in the other direction. The asymmetrical deposition caused by the magnetic lines parallel to the direction of rotation that activates the gaseous plasma ions to follow a direction perpendicular to the direction of rotation is neutralized by the rotation of the magnetron device. However, the magnetic lines perpendicular to the direction of rotation lead to an asymmetric deposition in a direction parallel to the direction of rotation. FIGS. 1A and 1B are diagrams showing a thin film on an opening section within a lithographic mark or overlay mark on a wafer using a conventional D.C. magnetron sputtering method. Due to the presence of concentric openings on the target and the effect of the magnetic lines on the sputtering angle of gaseous plasma ions relative to the bombarding target, the film 102 on the wafer 100 may be asymmetrically deposited on the sidewalls of the openings 104. Consequently, there will be a rotational shift (labeled 106) or radial shift (labeled 108) in the coordinates of the overlay mark.
In general, overlay mark measurement is based on the height difference on the surface of a wafer as measured by the difference in brightness at various interfaces. When a metallic film on the sidewall of an opening is asymmetrically deposited, the mid-point obtained by gauging the height difference of opening sidewalls may deviate from the true value. Furthermore, the degree of asymmetrical deposition will increase in proportional to the target consumption and hence the degree of shifting will increase with use. Although the shifting problem in lithographic process can be rectified through a few adjusting steps, this is not an efficient means because each depositing station and the circumstance around each shifting event are different.